The present invention relates to a digital filter device and a method for processing TV composite signals, and more particularly to digital filters for processing an audio component and a video component of a TV composite signal.
The fundamental function of a video decoder is to separate the video component from the audio component. The separation of the video and audio components can be performed in multiple ways. Many separation methods have been implemented since the introduction of television.
A color television (TV) composite signal includes two basic components: the video component and the audio component. The video component further contains the luminance information, the chrominance information, and the synchronization information (Horizontal and Vertical Sync).
Television channels are grouped in multiple bands, such as low VHF band, high VHF band, and UHF band. These bands span from 40 MHz to 890 MHz. Within the frequency bands, adjacent channels are spaced at either 6 MHz or 8 MHz in most parts of the world.
For receiving a desired channel, a local oscillator of a TV receiver is tuned to a frequency so that after mixing, the desired or selected channel is produced at a preset intermediate frequency (IF) signal that is then further processed in a subsequent demodulator. IF demodulators typically employ surface acoustic wave (SAW) filters to condition the IF signal prior to extracting the video and the audio signals. The SAW filter rejects the energy bands associated with channels adjacent to the desired channel. The SAW filter provides a Nyquist slope bandpass response for the IF signal (see FIG. 1A). Since TV demodulators operate at either a 36 MHz or 44 MHz intermediate frequency, at least two SAW filters with center frequency at 36 MHz and at 44 MHz have to be used in order to support multi-standard TV signal reception.
Because a TV composite signal contains the luminance, chrominance and audio information, additional SAW filters for filtering the video component and audio component from the TV composite signal are required. Since different TV standards use different broadcast frequencies with different bandwidths, a traditional multi-standards TV receiver is required to have multiple different tuners equipped with video and audio filters for the decoders.
Further, even for one-standard (e.g. for one country) TV system, a demodulator requires to have at least two IF SAW filters, one for the video component and another one for the audio component in order to separate the video and audio components within the TV composite signal. The need of SAW filters not only requires a large area for layouts, but also increases the costs of the PCB bill of materials. For applications in handheld or wearable TV tuners, a small PCB layout area is considered especially critical. And the portability requirement imposes that TV tuners support multiple standards because, as consumers are now traveling to different countries around the world, they expect to receive local TV programs using their laptops, notebooks, smart phones, and other devices.
It can be seen that there is a need of a filter device and a method that can perform multi-standard channel filtering without resorting to the use of any mechanical IF SAW filters in order to reduce part count, cost, and size, and increase system reliability.
Accordingly, the present invention provides a technical solution to the problems discussed above, and that this solution can be economically integrated into a single electronic device using standard CMOS or any other integrated circuit processes.